Variable-mechanical-impedance artificial legs

ABSTRACT

In one aspect, the invention provides methods and apparatus facilitating an adjustable-stiffness prosthesis or orthosis (including approximations to arbitrarily definable non-linear spring functions). Spring rates may be varied under no-load conditions during a walking gate cycle to minimize power consumption. In another aspect, the invention provides methods and apparatus for outputting positive power from a prosthesis or orthosis, facilitating high-performance artificial limbs. In one embodiment of the invention, the positive power is transferred from a functioning muscle to the prosthesis or orthosis, which mimics or assists a non-functioning or impaired muscle. In another embodiment of the invention, the positive power comes from an on-board power source in the prosthesis or orthosis.

[0001] This patent application claims priorety of Provisional PatentApplication No. 60/395,938, filed Jul. 15, 2002.

[0002] The invention relates generally to the fields of legged robotics,orthotic leg devices and prosthetic leg joints, and more specifically toartificial limbs with time-variable mechanical parameters.

BACKGROUND

[0003] Prosthetic limbs have come a long way since the days of simplewooden “peg legs”. Today, amputee men running on a prosthetic leg canbeat race times of the best unimpaired women runners. It is believedthat new advances in prosthetic limbs (such as those embodied in thepresent invention) will soon lead to amputees being able to out-performthe best unimpaired athletes of the same sex in sports such as running.It is an object of the present invention to advance the state ofprosthetic limbs to a new level, providing increased athleticperformance, increased control, and reduced body strain. It is a furtherobject of the present invention to provide essential elements needed formaking prosthetic limbs that more accurately mimic the mechanicalbehavior of healthy human limbs.

[0004] Description of Normal, Level-ground Walking:

[0005] In order to establish terminology used in this document, thebasic walking progression from heel strike to toe off is firstexplained. There are three distinct phases to a walking stance-period asdepicted in FIG. 1 with heel-toe sequence 1 through 7.

[0006] Saggital Plane Knee Phases

[0007] 1. Beginning with heel strike, the stance knee begins to flexslightly (Sequence 1-3). This flexion allows for shock absorption uponimpact as well as keeping the body's center of gravity at a moreconstant vertical level throughout stance.

[0008] 2. After maximum flexion is reached in the stance knee, the jointbegins to extend again, until full extension is reached (Sequence 3-5).

[0009] 3. During late stance, the knee of the supporting leg begins toflex again in preparation for the swing phase (Sequence 5-7). This isreferred to in the literature as “knee break”. At this time, theadjacent foot strikes the ground and the body is in “double supportmode” (that is to say, both legs are supporting body weight).

[0010] Saggital Plane Ankle Phases

[0011] 1. Beginning with heel strike, the ankle undergoes a controlledplantar-flexion phase where the foot rotates towards the ground untilthe forefoot makes contact (Sequence 1-2).

[0012] 2. After controlled plantar-flexion, the ankle undergoes acontrolled dorsi-flexion phase where the tibia rotates forwardly whilethe foot remains in contact with the ground (Sequence 2-5).

[0013] 3. During late stance, the ankle undergoes a poweredplantar-flexion phase where the forefoot presses against the groundraising the heel from the ground (Sequence 5-7). This final phase ofwalking delivers a maximal level of mechanical power to the walking stepto slow the fall of the body prior to heel strike of the adjacent,forwardly positioned leg.

[0014] The development of artificial leg systems that exhibit naturalknee and ankle movements has been a long standing goal for designers oflegged robots, prostheses and orthoses. In recent years, significantprogress has been made in this area. The current state-of-the-art inprosthetic knee technology, the Otto Bock C-Leg, enables amputees towalk with early stance knee flexion and extension, and thestate-of-the-art in ankle-foot systems (such as the Össur Flex-Foot)allow for ankle controlled plantar-flexion and dorsi-flexion. Althoughthese systems restore a high level of functionality to leg amputees,they nonetheless fail to restore normal levels of ankle poweredplantar-flexion, a movement considered important not only for biologicalrealism but also for walking economy. In FIG. 2, ankle power data areshown for ten normal subjects walking at four walking speeds from slow(½ m/sec) to fast (1.8 m/sec). As walking speed increases, both positivemechanical work and peak mechanical power output increase dramatically.Many ankle-foot systems, most notably the Flex-Foot, employ springs thatstore and release energy during each walking step. Although some powerplantar-flexion is possible with these elastic systems, normalbiological levels are not possible. In addition to power limitations,the flex-foot also does not change stiffness in response todisturbances. The human ankle-foot system has been observed to changestiffness in response to forward speed variation and groundirregularities. In FIG. 3, data are shown for a normal subject walkingat three speeds, showing that as speed increases ankle stiffness duringcontrolled plantar-flexion increases.

[0015] Artificial legs with a mechanical impedance that can be modeledas a spring in parallel with a damper are known in the art. Someprostheses with non-linear spring rates or variable damping rates arealso known in the art. Unfortunately, any simple linear or non-linearspring action cannot adequately mimic a natural limb that puts outpositive power during part of the gait cycle. A simple non-linear springfunction is monotonic, and the force vs. displacement function is thesame while loading the spring as while unloading the spring. It is anobject of the present invention to provide actively electronicallycontrolled prosthetic limbs which improve significantly on theperformance of artificial legs known in the art, and which requireminimal power from batteries and the like. It is a further object of thepresent invention to provide advanced electronically-controlledartificial legs which still function reasonably well should the activecontrol function fail (for instance due to power to the electronics ofthe limb being lost). Still further, it is an object of the presentinvention to provide artificial legs capable of delivering power atplaces in the gait cycle where a normal biological ankle delivers power.And finally, it is an object of the present invention to provideprosthetic legs with a controlled mechanical impedance and the abilityto deliver power, while minimizing the inertial moment of the limb aboutthe point where it attaches to the residual biological limb.

[0016] During use, biological limbs can be modeled as a variablespring-rate spring in parallel with a variable damping-rate damper inparallel with a variable-power-output forcing function (as shown in FIG.4a). In some activities, natural human limbs act mostly as spring-dampercombinations. One example of such an activity is a slow walk. Whenwalking slowly, a person's lower legs (foot and ankle system) act mostlyas a system of springs and dampers. As walking speed increases, theenergy-per-step put out by the muscles in the lower leg increases. Thisis supported by the data in FIG. 2.

[0017] Muscle tissue can be controlled through nerve impulses to providevariable spring rate, variable damping rate, and variable forcingfunction. It is an objective of the present invention to better emulatethe wide range of controllability of damping rate, spring rate, andforcing function provided by human muscles, and in some cases to providecombination of these functions which are outside the range of naturalmuscles.

SUMMARY OF THE INVENTION

[0018] There are two major classes of embodiments of the presentinvention. The first major class provides for actively controlledpassive mechanical parameters (actively controlled spring rate anddamping rate). This major class of embodiments will be referred to asvariable-stiffness embodiments. Three sub-classes of variable-stiffnessembodiments are disclosed:

[0019] 1) Multiple parallel interlockable springs.

[0020] 2) Variable mechanical advantage.

[0021] 3) Pressure-variable pneumatics.

[0022] The second major class of embodiments of the present inventionallows for the controlled storage and release of mechanical energywithin a gait cycle according to any arbitrary function, includingfunctions not available through simple nonlinear springs. Within thissecond major class of embodiments, energy can be stored and released atrates which are variable under active control. Thus for a given joint,the force vs. displacement function is not constrained to be monotonicor single-valued. Within this class of embodiments, energy (from eithermuscle or a separate on-board power source) can be stored and releasedalong arbitrarily defined functions of joint angular or lineardisplacement, force, etc. This major subclass of embodiments shall bereferred to herein as energy transfer embodiments. Two sub-classes ofenergy transfer embodiments are disclosed:

[0023] 1) Bi-articular embodiments (which transfer energy from aproximal joint to a distal joint to mimic the presence of a missingjoint).

[0024] 2) Catapult embodiments (which store energy from a power sourceover one span of time and release it over another span of time to aidlocomotion).

[0025] The present invention makes possible prostheses that havemechanical impedance components (damping and spring rate) and poweroutput components that are actively controllable as functions of jointposition, angular velocity, and phase of gait. When used in a prostheticleg, the present invention makes possible control of mechanicalparameters as a function of how fast the user is walking or running, andas a function of where within a particular step the prosthetic leg isoperating.

[0026] It is often necessary to apply positive mechanical power inrunning shoes or in orthotic and prosthetic (O&P) leg joints to increaselocomotory speed, to jump higher, or to produce a more natural walkingor running gait. For example, when walking at moderate to high speeds,the ankle generates mechanical power to propel the lower leg upwards andforwards during swing phase initiation. In FIG. 2, data are shown forten normal subjects showing that the ankle delivers more energy during asingle step than it absorbs, especially for moderate to fast walkingspeeds.

[0027] Two catapult embodiments of the present invention are describedin which elastic strain energy is stored during a walking, running orjumping phase and later used to power joint movements. In a firstembodiment, catapult systems are described in which storage and releaseof stored elastic energy occurs without delay. In a second embodiment,elastic strain energy is stored and held for some time period beforerelease. In each Embodiment, mechanism architecture, sensing and controlsystems are described for shoe and O&P leg devices. Although just a fewdevices are described herein, it is to be understood that the principlescould be used for a wide variety of applications within the fields ofhuman-machine systems or legged robots. Examples of these first andsecond catapult embodiments are shown in FIGS. 4 through 6.

[0028] One bi-articular embodiment of the invention described hereincomprises a system of knee-ankle springs and clutches that afford atransfer of energy from hip muscle extensor work to artificial anklework to power late stance plantar-flexion. Since the energy for ankleplantar-flexion originates from muscle activity about the hip, a motorand power supply need not be placed at the ankle, lowering the totalmass of the knee-ankle prosthesis and consequently the metabolic costassociated with accelerating the legs in walking. Examples of theseembodiments are shown in FIGS. 7 and 8.

[0029] Several variable-stiffness embodiments are described herein inwhich variable spring-rate structures are constructed by varying thelength of a moment arm which attaches to a spring element about a pivotaxis, thus providing a variable rotational spring rate about the pivotaxis. Examples of such embodiments are depicted in FIGS. 9 through 11.In a preferred embodiment, variations in the length of the moment armare made under microprocessor control at times of zero load, to minimizepower consumed in the active control system.

[0030] Variable-stiffness embodiments of the present invention employingmultiple interlockable parallel spring elements are depicted in FIGS. 12through 14. In FIGS. 12a and 12 b, multiple parallel elastic leaf springelements undergo paired interlocking at pre-set joint flexures or undermicroprocessor control. This embodiment makes possible arbitrarypiecewise-linear approximations to non-linear spring functions (such asfunction 624 in FIG. 12d). A pneumatic embodiment which can beconfigured to behave similarly to the leaf spring embodiments shown inFIGS. 12a and 12 b is shown in FIG. 13. In the pneumatic embodiment ofFIG. 13, valves are electronically closed to effectively increase thenumber of pneumatic springs in parallel.

[0031] The multiple parallel spring elements in FIGS. 12a, 12 b, andFIG. 13 could equivalently be replaced by other types of springelements, such as coil springs, torsion bars, elastomeric blocks, etc.

BRIEF DESCRIPTION OF THE FIGURES

[0032]FIG. 1: Depiction of stages of a gait cycle, including controlledplantar-flexion, controlled dorsi-flexion, and powered plantar-flexion.

[0033]FIG. 2: Data from ten normal subjects are plotted showingmechanical power output versus percent gait cycle in walking. Both zeroand one hundred percent gait cycle correspond to heel strike of the samefoot

[0034]FIG. 3: Data for one subject, showing normal biological anklefunction during the controlled plantar-flexion phase of walking.

[0035]FIG. 4a: Basic catapult embodiment of the present invention,represented in terms of a lumped-parameter model.

[0036]FIG. 4b: Force-displacement graph where darkened area representsextra stored energy (used in walking/running) put into catapult systemby force actuator while prosthetic foot is off the ground.

[0037]FIG. 4c: Side view of simplified prosthetic mechanism designed toprovide powered plantar-flexion.

[0038]FIG. 4d: Front view of simplified prosthetic mechanism designed toprovide powered plantar-flexion.

[0039]FIG. 5a: Catapult foot prosthesis or shoe orthosis for walking,running, and jumping, shown in the equilibrium configuration.

[0040]FIG. 5b: Catapult foot prosthesis or shoe orthosis for walking,running, and jumping, shown in a compressed state.

[0041]FIG. 6a: Side view of catapult leg prosthesis for walking,running, and jumping, shown in the equilibrium state.

[0042]FIG. 6b: Side view of catapult leg prosthesis for walking,running, and jumping, shown in a compressed state.

[0043]FIG. 6c: Front view of catapult leg prosthesis for walking,running, and jumping.

[0044]FIG. 7: An external, bi-articular transfemoral prosthesis ororthosis is shown in a heel strike to toe-off walking sequence. Thesystem comprises springs and controllable clutches to transfer energyfrom hip muscular work to ankle powered plantar-flexion work.

[0045]FIG. 8: An external, bi-articular transfemoral prosthesis ororthosis is shown in a heel strike to toe-off walking sequence. Thesystem comprises pneumatic springs and controllable valves to transferenergy from hip muscular work to ankle powered plantar-flexion work.

[0046]FIG. 9: Perpendicularly-variable-moment pivotal spring structure.

[0047]FIG. 10: Mechanical diagram of a low-profile prosthetic foot wherespring elements are actively controlled (positioned) to affect anklejoint stiffness.

[0048]FIG. 11: Variable-stiffness joint according to the presentinvention, utilizing variable mechanical advantage to produce variablespring rate and/or variable damping rate.

[0049]FIG. 12a: Multiply interlockable parallel leaf spring structure,shown in equilibrium position.

[0050]FIG. 12b: Multiply interlockable parallel leaf spring structure,shown in a stored-energy position.

[0051]FIG. 12c: End view of two dove-tailed slidably attached leafspring terminations with controllable interlock actuator.

[0052]FIG. 12d: Piecewise-linear approximation to nonlinear springfunction achieved by interlocking successive parallel leaf springs atvarious angles, and smoothed nonlinear spring function achieved byinterlocking successive parallel leaf springs through coupling springs.

[0053]FIG. 12e: Nonlinear damping element coupling mechanism forcoupling multiple spring elements.

[0054]FIG. 13: Multiple-pneumatic-chamber variable spring rate andenergy transfer system.

[0055]FIG. 14: Prosthetic ankle/foot utilizing multiple interlockableparallel leaf springs for ankle spring.

[0056]FIG. 15: Example prosthetic ankle/foot known in the art.

[0057]FIG. 16: Variable-stiffness pneumatic spring.

DETAILED DESCRIPTION

[0058] A powered-catapult embodiment of the present invention is shownin FIGS. 4a-4 d. FIG. 4a is a lumped-element model of a powered-catapultprosthetic. The mounted end 203 of the prosthesis attaches to the body,and the distal end 204 of the prosthesis interfaces to the environment(such as the ground for a leg prosthesis). Mounted end 203 is coupled todistal end 204 through spring 202, and through the series combination offorce actuator 205 and force sensor 201. In some embodiments,displacement sensor 206 may also be included in parallel with spring202. If the system is designed to operate in parallel with an existinglimb, the muscles of the existing limb are modeled by muscle 200.

[0059] A mechanical implementation of lumped-element diagram 4 a isshown in side view in FIG. 4c and in front view in FIG. 4d. In apreferred embodiment, during the portion of a gait cycle when the footis not in contact with the ground, motor 205 turns spool 209 to wind onsome of tension band 208, storing energy in spring 202. Force sensor 201and winding distance sensor 207 may be used in a control loop to controlhow much energy is stored in spring 202, and how rapidly this energy isstored. Once the desired energy has been stored, clutch 207 is actuatedto keep tension band 208 from unwinding and spring 202 from relaxinguntil the control system decides to release the stored energy. Theenergy stored in spring 202 during the swing phase of the gait cycle isrepresented by the dark area on the force vs. distance graph shown inFIG. 4b.

[0060] During the powered plantar-flexion phase of the gait cycle, thecontrol system releases clutch 207, allowing the stored energy in spring202 to be released, imitating the powered plantar-flexion stage of anormal gait cycle. This release of energy mimics the pulse of power putout by a biological ankle during the powered plantar-flexion stage of awalking or running gait cycle.

[0061] In an alternate embodiment, motor 205 may store energy in spring202 at the same time as the natural leg stores impact energy during thegait cycle. This embodiment can be used to effectively implement onespring rate during compression (such as the spring rate depicted by theline from the origin to point Kd in FIG. 4b) and another spring rateduring release (such as the spring rate depicted by the line from theorigin to point Ks in FIG. 4b).

[0062] In an alternate embodiment, FIG. 5 shows a prosthetic foot orshoe orthosis that stores both muscle energy and motor energy in springmechanism 300 during the gait cycle, for release during the poweredplantar-flexion stage of the walking gait cycle (toe-off propulsion).When walking on this type of catapult prosthesis or foot orthosis, aperson would experience a first (lower) spring rate (depicted by theline from the origin to point Kd in FIG. 4b), and a second (higher)spring rate (depicted by the line from the origin to point Ks in FIG.4b) when releasing energy from spring 300 during the poweredplantar-flexion phase of the gait cycle.

[0063] For catapult embodiments depicted in both FIG. 4 and in FIG. 5,part of the energy released during powered plantar-flexion came from legmuscle action compressing springs 202 and 300, and part came from anelectromechanical actuator such as a motor. In a preferred embodiment ofthe present invention as depicted in FIG. 4, the majority of powerstored in spring mechanisms by electromechanical actuators occurs duringthe minimal-load portion of the walking/running gait cycle (swingphase), and the start of the energy-release phase (late stance phase) ofthe gait cycle may be time-delayed with respect to the swing phase whenmotor energy is stored.

[0064]FIG. 6 is another depiction of the catapult leg prosthesis of FIG.4, also showing socket 400, which attaches to the residual biologicallimb. Although the leg prostheses shown in FIGS. 4 and 6 arebelow-the-knee prostheses, the invention could also be employed inabove-knee prostheses.

[0065] Two bi-articular embodiments of the present invention are shownin FIGS. 7 and 8. In a first embodiment (FIG. 7), a prosthesis (above orbelow knee), robotic leg or full leg orthosis is shown having above-kneesegment (a), knee joint (b), ankle joint (c), posterior knee pivot (d),posterior clutch (e), posterior spring (f), posterior cord (g),knee-ankle transfer clutch (h), anterior pivot (i), anterior clutch (j),anterior spring (k), and anterior cord (l). Anterior spring (k)stretches and stores energy during early stance knee flexion (from 1 to3) and then releases that energy during early stance knee extension(from 3 to 5). Here spring (k) exerts zero force when the knee is fullyextended, and anterior clutch (j) is engaged or locked throughout earlystance knee flexion and extension (from 1 to 5). This stored energy,together with an applied extensor hip moment from either a robotic orbiological hip, result in an extensor moment at the knee, forcing theknee to extend and stretching posterior spring (f) (from 3 to 5). Thespring equilibrium length of posterior spring (f) is equal to theminimum distance from posterior knee pivot (d) to posterior clutch (e)(leg configuration 3 in FIG. 7). To achieve this spring equilibrium,posterior clutch (e) retracts posterior cord (g) as the distance fromposterior knee pivot (d) to posterior clutch (e) becomes smaller. Whenthis distance begins to increase in response to knee extension and ankledorsi-flexion (from 4 to 5), posterior clutch (e) engages, causingposterior spring (f) to stretch. When the ankle is maximallydorsi-flexed and the knee fully extended (leg configuration 5),posterior spring (f) becomes maximally stretched. When the leg assumesthis posture, knee-ankle transfer clutch changes from a disengaged stateto an engaged state. Engaging the knee-ankle clutch mechanically groundsspring (f) below the knee rotational axis, and consequently, all theenergy stored in spring (f) is transferred through the ankle to powerankle plantar-flexion (from 6 to 7). During late stance (from 5 to 6),the knee of the supporting leg begins to flex again in preparation forthe swing phase. For this late stance knee flexion, anterior clutch (j)is disengaged to allow the knee to freely flex without stretchinganterior spring (k).

[0066] It should be understood that the bi-articular knee-ankleinvention of embodiment I (FIG. 7) could assume many variations as wouldbe obvious to those of ordinary skill in the art. For example, thesystem described herein could act in parallel to additional ankle-footsprings and/or to an active or passive knee damper. Additionally,instead of mechanically grounding spring (f) distal to the knee axis toeffectively transfer all the stored energy through the ankle, theperpendicular distance from the line of spring force (f) to the knee'saxis of rotation could go to zero as the knee approaches full extension.

[0067] In a second embodiment (FIG. 8), a prosthesis (above or belowknee), robotic leg or full leg orthosis is shown having a similar energytransfer from hip muscle extensors to artificial leg to power ankleplantar-flexion, accept energies are stored within pneumatic springsabout the knee and then transferred to the ankle via a fluid transfersystem. In this embodiment, the transfer of energy occurs without aphysical bi-articular spring such as posterior spring (f) in FIG. 7. Inthis embodiment, anterior pneumatic spring (j) compresses and storesenergy during early stance knee flexion (from 1 to 3). Here anteriorknee valve (k) is closed or locked throughout early stance knee flexionand extension (from 1 to 5). This stored energy, together with anapplied extensor hip moment from either a robotic or biological hip,result in an extensor moment at the knee, forcing the knee to extend andcompress posterior pneumatic spring (f) (from 3 to 5). It is importantto note that posterior knee valve (g) is open during early stance kneeflexion so that posterior pneumatic spring (f) exerts little force. Kneevalve (g) is then closed during knee extension so that energy is storedin the posterior pneumatic spring (f). When the ankle is maximallydorsi-flexed and the knee fully extended (leg configuration 5),posterior pneumatic spring (f) is maximally compressed. When the legassumes this posture, knee-ankle transfer valve changes from a closedstate to an open state, and anterior ankle valve (n) changes to a closedstate, allowing all the energy stored in spring (f) is be transferredthrough the ankle to power ankle plantar-flexion (from 6 to 7). Duringlate stance (from 5 to 6), the knee of the supporting leg begins to flexagain in preparation for the swing phase. For this late stance kneeflexion, anterior and posterior valves (g, k) are open to allow the kneeto freely flex without compressing anterior spring (j).

[0068] It should be understood that the bi-articular knee-ankleinvention of embodiment II (FIG. 8) could assume many variations aswould be obvious to those of ordinary skill in the art. For example, thesystem described herein could act in parallel to active or passiveankle-foot springs and/or to an active or passive knee damper.Additionally, the energy in posterior pneumatic spring (f) could betransferred to a temporary holding chamber to be later released to theankle during powered plantar-flexion.

[0069] The mechanical system in FIG. 9 is avariable-mechanical-advantage embodiment of a variable-stiffness spring.Motors 500 and motor-driven screws 505 serve to change the moment ofcompression of bow spring 503 about pivot point 504. This mechanism maybe used to adjust spring stiffness with minimal power under no-loadconditions. It may also be used as an alternative way of storing energyin a spring which is under load, and thus may be used as a component ofan immediate-release catapult system such as depicted in FIG. 5.

[0070]FIG. 10 depicts a low-profile prosthetic foot-ankle with top plate1 and bottom plate 2, where spring elements are actively controlled(positioned) to affect ankle joint stiffness. This embodiment of thepresent invention is a variable-stiffness embodiment of the “variablemechanical advantage” sub-class. In this low-profile prosthetic anklejoint embodiment, side-to-side spring rates of the prosthetic ankle andfront-to-back spring rates of the prosthetic ankle are adjusted byvarying the distance of spring elements 4, 5, 6, and 7 from the centralpivot point 15 of the ankle joint. Spring top plates 13 and springbottom plates 12 of spring elements 4, 5, 6, and 7 slide in tracks 14,driven by position-adjusting motors 8, 9, 10, and 11. In a preferredembodiment, motors 8, 9, 10, and 11 only change the positions of springelements 4, 5, 6, and 7 when the ankle joint is under zero load (forinstance, during the part of the walking gait when the foot is not incontact with the ground). Adjustment of spring position under zero loadallows position adjustments to be done with minimal energy. Thisembodiment offers independent inversion/eversion stiffness control aswell as independent plantar-flexion and dorsi-flexion control.

[0071] A variable stiffness ankle-foot prosthesis embodiment accordingto the present invention is shown in FIG. 11. Constant-rate spring ordamping element 1700 fixedly attached at one end and movably attached atthe other end. Attachment point 1701 may be moved in and out withrespect to the effective pivot point of the ankle joint. If element 1700is a damping element, this configuration provides a variable dampingankle joint. If element 1700 is a spring element, this configurationprovides a variable spring rate ankle joint. FIGS. 9, 10 and 11demonstrate how a constant element can be transformed into a variableelement according to the present invention, by varying mechanicaladvantage. In non-catapult preferred embodiments of the presentinvention, the variation in mechanical advantage takes place such thatthe motion used to vary the mechanical advantage takes placesubstantially perpendicular to the force the element being moved isunder, thus minimizing the work needed to vary the mechanical advantage.

[0072]FIGS. 12a and 12 b depict a multiple-parallel-leaf-springembodiment of a variable mechanical impedance according to the presentinvention. Leaf springs 600 are bound together and bound tightly toattaching bracket 602 at one end by bolt 601. At the other end, leafsprings terminate in slidably interlocking blocks 603, which may belocked together dynamically in pairs by interlocking plates 605. Eachinterlocking plate 605 is permanently bonded to one leaf springterminator block 603 at surface interface 606, and controllably bindableto a second leaf spring terminator block 604 at a second interface 607,by binding actuator 608. Binding actuator 608 may bind surface interface607 by any number of means such as mechanical clamp, pin-in-socket,magnetic clamp, etc. Adjacent leaf spring terminator blocks are slidablyattached by dovetail slides or the like. The structure shown in FIGS.12a-c can be used to implement a piecewise-linear spring function suchas function 604 depicted in FIG. 12d, by engaging successive interlocks605 at pre-determined points in spring flexure, and disengaging at likepoints.

[0073] In a preferred embodiment, the slope discontinuities in function604 may be “smoothed” by coupling successive leaf springs throughcoupling springs. In FIG. 12d, stop plate 619 is affixed to leaf springtermination 620, and coupling spring 621 is mounted to leaf springtermination 618 through coupling spring mount 622. Leaf springtermination 620 is free to slide with respect to leaf spring termination618 until coupling spring 621 and stop plate 619 come in contact.Coupling spring 621 acts to smooth the transition from the uncoupledstiffness of two leaf springs to the coupled stiffness of two leafsprings, resulting in smoothed force-displacement function 625 in FIG.12d.

[0074] In a preferred embodiment, coupling spring 621 is itself a stiff,nonlinear spring. In another preferred embodiment, coupling spring 621may have actively controllable stiffness, and may be made according toany of variable-stiffness spring embodiments of the present invention.

[0075]FIG. 12e depicts a non-linear dissipative coupling mechanism forcoupling pairs of spring elements in a multiple-parallel-element spring.Mechanical mounts 609 and 610 affix to a pair of spring elements to becoupled. In a preferred embodiment, one of 609 and 610 is permanentlyaffixed and the other of 609 and 610 is controllably affixed through amechanism such as 608 described above. Piston 611 is coupled to mount609 through rod 612 which passes through seal 614. Thus piston 611 maymove back and forth in chamber 615 along the axis of rod 612. Chamber615 is preferably filled with viscose or thixotropic substance 616. Aviscose substance can be used in chamber 616 to provide a mechanicalcoupling force proportional to the square of the differential velocitybetween mounts 609 and 610. A thixotropic substance (such as a mixtureof corn starch and water) can be used to provide an even more nonlinearrelationship between coupling force and the differential velocitybetween coupling plates 609 and 610. Alternately, an electronicallycontrolled variable damping element may be used in series with forcesensor 617 between mounts 609 and 610, to provide an arbitrarynon-linear dissipative coupling.

[0076] Utilizing a nonlinear dissipative coupling between pairs ofelements in a multiple-parallel-element spring allows joint spring ratesin a prosthetic limb which are a function of velocity. Thus, a jointspring rate can automatically become stiffer when running than it iswhile walking.

[0077] In one preferred embodiment, chamber 615 is rigidly mounted tomount 610. In another preferred embodiment, chamber 615 is mounted tomount 610 through coupling spring 623. In a preferred embodiment,coupling spring 623 may be an actively-controlled variable stiffnessspring according to the present invention.

[0078]FIG. 13 depicts a multiple-couplable-parallel element pneumaticembodiment of the present invention. Multiple parallel pneumaticchambers 900 couple mounting plates 908 and 909. Pneumatic hoses 902connect chambers 900 to a common chamber 901 through individuallyactuatable valves 903. Spring stiffness between plates 908 and 909 ismaximized when all valves 903 are closed, and minimized when all valves903 are open. Additional pneumatic element 905 may be added to transferpower from one prosthetic joint to another.

[0079] In an immediate-energy-transfer embodiment of the presentinvention according to FIG. 13, valves 904 and 906 may be timed toactuate in sequence with valves 903 to transfer power directly fromchamber 905 to chambers 900. In a delayed-energy-transfer embodiment ofthe present invention according to FIG. 13, energy may be transferredfrom chamber 905 to chambers 900 or vice versa in a delayed manner, bychambers 900 or chamber 905 first pressurizing chamber 901, thenisolating chamber 901 by closing valves 903 and 904 for some period oftime, then transferring the energy stored in chamber 901 to chambers 900or 905 by opening the appropriate valves.

[0080]FIG. 15a depicts a prosthetic ankle-foot system known in the art.Ankle spring 1500 is affixed to foot-plate 1501. One variable-stiffnessembodiment of the present invention shown in FIG. 15 uses amultiple-parallelly-interlockable-leaf-spring structure such as thatshown in FIG. 12 in place of ankle spring 1500.Multiple-parallelly-interlockable-leaf-spring 1600 allows for differentspring rates in forward and backward bending, allowing separatelycontrollable rates of controlled plantar-flexion and controlleddorsi-flexion.

[0081] In one embodiment of the present invention (shown in FIG. 15b),ankle spring 1500 is split into inner ankle spring 1500 a, and outerankle spring 1500 b, and heel spring 1501 is split rearward ofattachment point AP into inner heel spring 1501 a and outer heel spring1501 b. In a preferred embodiment, ankle springs 1500 a and 1500 b andheel springs 1501 a and 1501 b each comprise actively-variablemulti-leaf springs such as ankle spring 1600 in FIG. 14. Having separateinner and outer variable-stiffness ankle springs allows for activecontrol of side-to-side stiffness of the prosthetic ankle joint. Havingseparate inner and outer variable-stiffness heel springs allows foractive control medio-lateral ankle stiffness.

[0082] A pneumatic embodiment of a variable-stiffness spring for aprosthesis is shown in FIG. 16. Male segment 702 comprises one end ofthe overall variable-stiffness spring, and female segment 701 comprisesthe other end. Control electronics 710 are contained in the upper end ofmale segment 710. Intake valve 715 is actuatable to allow air to enterpressure chamber 708 through air intake channel 716 when pressurechamber 708 is below atmospheric pressure (or an external pump may beused to allow air to enter even when chamber 708 is above atmosphericpressure). Air pressure sensor 709 senses the pressure in pressurechamber 708. Pressure chamber 708 is coupled to second pressure chamber703 through valve 711. The air in pressure chamber 703 acts as apneumatic spring in parallel with spring 704. Motor 705 turns ball screw707 to move piston 706 back and forth to control the volume of pressurechamber 708. Pressure in pressure chamber 703 may be lowered to adesired value by opening valve 703 for a controlled period of time,allowing air to escape through pressure release channel 714.

[0083] In one mode of operation, valve 711 is open and pressure chambers708 and 703 combine to form a single pressure chamber. In this mode,movement of piston 706 directly controls the overall pressure chambervolume, and thus the overall pneumatic spring rate. In another mode ofoperation, valve 711 is closed, and valve 706 may be opened and piston706 may withdrawn to add air to the system.

[0084] In a preferred embodiment of a variable-stiffness leg prosthesisaccording to the present invention is implemented through the pneumaticsystem of FIG. 16, motion of piston 706 occurs under minimal load, suchas during the phase of gait when the foot is off the ground, or when theuser is standing still.

[0085] The pneumatic system shown in FIG. 16 may also be used toimplement immediate-release or delayed-release catapult embodiments ofthe present invention. An immediate-release catapult may be implementedby opening valve 711, and using motor 705 to add power (for instance,during the powered plantar-flexion phase of gait) as the power isneeded. In a delayed-release catapult embodiment of the presentinvention, valves 715 and 711 are closed while motor 705 moves piston706 to pressurize chamber 708, and then energy stored in chamber 708 israpidly released during a phase of gait to produce the same effect aspowered plantar-flexion.

[0086] In a preferred embodiment of the present invention, a pneumaticprosthetic leg element according to FIG. 16 is combined with themultiple controllably-couplable parallel leaf spring prostheticankle-foot of FIG. 15 to provide a prosthetic limb which providespowered plantar-flexion, controllable compressional leg springstiffness, and controllable ankle stiffness during controlledplantar-flexion and controlled dorsi-flexion.

[0087] The foregoing discussion should be understood as illustrative andshould not be considered to be limiting in any sense. While thisinvention has been particularly shown and described with references topreferred embodiments thereof, it will be understood by those skilled inthe art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention as definedby the claims.

Having described the invention, what is claimed is:
 1. A variableimpedance prosthesis or orthosis, comprising: a. A proximal end forinterfacing to a user; b. a distal end for interfacing to theenvironment; c. a stiffness controller; d. a controllable-spring-ratespring element.
 2. The apparatus of claim 1, wherein saidcontrollable-stiffness spring element comprises multiple parallelinterlockable spring elements.
 3. The apparatus of claim 1, wherein saidcontrollable-stiffness spring element comprises a spring element with avariable mechanical advantage.
 4. The apparatus of claim 1, wherein saidcontrollable-stiffness spring element comprises multiple parallel valvedpneumatic spring elements.
 5. The apparatus of claim 1, wherein saidcontrollable-stiffness spring element comprises a spring element and aparallel powered mechanical force source.
 6. The apparatus of claim 1,wherein said controllable-stiffness spring element comprises a springelement and a series powered mechanical displacement source.
 7. Theapparatus of claim 1, wherein said controllable-spring-rate springelement further comprises: a. a first spring element disposed betweensaid proximal end and said distal end; b. a mechanical energy storageelement; c. a controllable power source configured to store energy insaid energy storage element; d. a controllable coupling between saidenergy storage element and said first spring element; e. a controllerconfigured to control timing and rate of power output of saidcontrollable mechanical power source, and coupling of controllablecoupling.
 8. The apparatus of claim 7, wherein said controllablemechanical power source comprises a muscle and a controllable mechanicalcoupling between said muscle and said energy storage element
 9. A methodfor providing variable mechanical impedance in a prosthetic or orthotic,comprising varying the spring rate a controllable-spring-rate springautomatically with a spring-rate controller as a function of a repeatedcycle of use of said prosthetic or orthotic.
 10. The method of claim 9,wherein said variable-spring-rate spring comprises multiple parallelinterlockable spring elements, and said controller controls theinterlocking of said elements.
 11. The method of claim 9, wherein saidvariable-spring-rate spring further comprises a first spring and anenergy storage element, and further comprising: a. storing energy from apower source in said energy storage element during a first span of time;b. releasing energy from said energy storage element in the form ofmechanical work displacing a proximal end of a prosthesis from a distalend of said prosthesis or orthosis during a second span of time.